MIDI_AUTODETECT      - let Allegro pick a MIDI sound driver
      MIDI_NONE            - no MIDI sound
      MIDI_CORE_AUDIO      - CoreAudio MIDI synthesizer (OS >= X.2 required)
      MIDI_QUICKTIME       - QuickTime Music Note Allocator MIDI output
      MIDI_DIGMID          - sample-based software wavetable player